The present invention concerns an apparatus for monitoring and controlling the tire inflation pressure in a tire testing machine.
In one form of such an apparatus, as disclosed for example in U.S. Pat. No. 2,695,520, the pneumatic tire to be tested is mounted in a tire testing machine between first and second measuring rim portions which are rotatable about a common axis and which are also displaceable axially relative to each other. The tire to be tested is thus carried between the first and second measuring rim portions and can be inflated therebetween. For that purpose, compressed air for inflating the tire is supplied from a stationary compressed air source and passes by way of a compressed air conduit into a pressure inflation chamber defined by the interior of the tire mounted between the first and second measuring rim portions, and the intermediate space between the first and second measuring rim portions. The compressed air conduit in that arrangement extends coaxially through a spindle which is connected to and rotates with one of the measuring rim portions. The compressed air is supplied to the outer end of the spindle by way of a rotary connection and thereby is passed into the compressed air conduit which extends through the spindle. The apparatus further includes a measuring means which is disposed in stationary outside members of the installation and which regulates the required inflation pressure in the tire to be tested. The entire assembly consisting of the air pressure control and regulating means is disposed upstream of the rotary connection to the spindle, in a part of the installation which is stationary.
Another apparatus for monitoring and controlling tire inflation pressure in a tire testing machine is disclosed in U.S. Pat. No. 4,702,287 and involves an arrangement in respect of the air pressure control and regulating means which is similar to that just discussed above.
During the operation of testing a pneumatic tire which, during the testing run, rolls against a testing drum or other suitable support surface, the tire inflation pressure must be kept substantially constant in order to prevent measurement errors from occurring. Fluctuations in tire inflation pressure give rise to fluctuations in the radial forces involved in rotation of the tire, and those fluctuations in radial forces can falsify the measurement result. The reason for that is that the tire contact area or contact patch, with which the tire bears against the rotating testing drum or other surface, fluctuates in size if the level of inflation pressure of the tire varies, thus giving rise to the above-mentioned fluctuations in radial forces. In the apparatuses discussed above, it is not possible to prevent air from leaking from the tire during the testing operation, so that the inflation pressure of the tire varies accordingly, such air leakage occurring at the sealing locations between stationary outside components of the installation and the rotating parts thereof, more particularly in the region of the rotary connection at the outer end of the spindle. However that leakage of air is not constant but depends on the speed and direction of rotation of the tire during the testing operation. For that reason it is necessary to provide for adjustment of the inflation pressure by means of the introduction of compressed air into the tire to set the inflation pressure to the correct level, in order thereby to compensate for the leakage losses and to maintain a constant pressure in the tire. However that is not a viable option in many situations, within a predetermined testing time. Furthermore, the existence of the above-mentioned leakage losses means that it is not generally possible to arrive at satisfactory conclusions regarding the conditions in respect of pressure in the rotating part of the apparatus, more especially in the pressure chamber defined between the first and second measuring rim portions and the interior of the tire.
Another difficulty arises out of the fact that a tire which suffers from radial run-out, in conjunction with a fluctuation in inflation pressure, will produce a fluctuation in the radial forces acting by way of the tire contact patch. Steps to eliminate the fluctuation in pressure by regulating the tire inflation pressure would result in the fluctuation in radial forces at least partially disappearing. That however simulates a uniform and true-running tire, which is not actually the case when the tire is brought into use on a vehicle.